A symmetrical configuration of second order system is already in use for small high pressure pumps; it has been observed, however, that the power required to move the totally unbalanced vanes and the amount of heat losses due to the friction of the vanes on the interior profile of the casing, or stator, are such that the system has a low efficiency.
Several fluid displacing machines are known where partially unbalanced vanes are used, some of which may be found described in U.S. Pat. No. 1,046,791 issued Dec. 10, 1912, U.S. Pat. No. 1,977,780 issued Oct. 23, 1934, U.S. Pat. No. 2,452,471 issued Oct. 26, 1948 and U.S. Pat. No. 3,642,390 issued Feb. 15, 1972.
These systems use a third order configuration or a trilobic housing around a circular rotor. Although the centrifugal action on the vanes and the heat losses on the housing may be reduced, the power required to accelerate the double vanes a number of times around the generating circle is such that it precludes the utilization of any of these systems.
Monolobic rotary air compressors which have a housing with an interior profile of a first order configuration are known but most of them make use of totally unbalanced vanes (single vanes) to separate the various compression chambers. These systems require elaborate lubrication sub-systems to reduce the friction on the housing and to ensure adequate sealing between the chambers. Indeed, these compressors require, for proper operation, large quantities of oil to reduce the effect of the pressure of the totally unbalanced vanes on the housing and further require an oil-recuperating sub-system to clean the compressed air of oil vapor and droplets; such sub-system is expensive and introduces substantial pressure losses that lowers the total efficiency of the system.